Question

Identify the combination of compounds that undergo Aldol condensation followed by dehydration to produce but-2-enal

• A. methanal and ethanal
• B. two moles of ethanal
• C. methanal and propanone
• D. ethanal and propanone
• E. two moles of ethanol

Hint:

Aldol products always have a total carbon count equal to the sum of the reactants. If you need a 4-carbon product, look for a 2+2 or 1+3 combination first to save time!

Answer 1

The Correct Option is B

Concept: Aldol condensation involves the reaction of two carbonyl molecules (with $\alpha$-hydrogens) in the presence of a dilute base to form a $\beta$-hydroxy aldehyde/ketone, which then dehydrates to an $\alpha,\beta$-unsaturated carbonyl.
• But-2-enal: Also known as Crotonaldehyde, it has 4 carbons and an aldehyde group.
• Retro-synthesis: To find the starting material, "cut" the double bond of $CH_3-CH=CH-CHO$.

Step 1: Analyze the target structure. But-2-enal structure: $CH_3-CH=CH-CHO$. It contains 4 carbons. Option (A) has 1+2 = 3 carbons. Option (C) has 1+3 = 4 carbons, but methanal has no $\alpha$-H. Option (B) has 2+2 = 4 carbons.

Step 2: Verify the condensation of Ethanal. Two moles of ethanal ($CH_3CHO$): 1. $CH_3CHO + CH_3CHO \xrightarrow{dil. NaOH} CH_3-CH(OH)-CH_2-CHO$ (3-hydroxybutanal) 2. On heating (dehydration): $CH_3-CH(OH)-CH_2-CHO \xrightarrow{-H_2O} CH_3-CH=CH-CHO$ This perfectly matches But-2-enal.